Developing add-ons isn't the hard part of making money out of them.
From personal experience of running a popular website that does around 1,300,000 page views a month:
Donations, forget it, a TINY percentage of people that use something will donate to use it. Think of the number of times you've donated for plugins or websites (did you donate to LQ?) and then half it and you'll get how much the average user donates. You'll need to have a total KILLER add-on that becomes essential if you wish to monetise it by charging for downloads. Alternatively you could highly monetise the website where the plugin is downloaded from and hope you make money that way. If you make upwards of $1 per 1,000 page views you'll be doing well in terms of eCPM revenue.
Then you have to MARKET your add-on. Word of mouth and posting on one or two forums may get you some installs but you'd probably have to invest in AdWords/Facebook advert campaigns.
Oh, and then be prepared for all the "The Internet Should Be FREE!!!! Why Isn't Your Add-On Free too????" BullCrap from people that don't actually understand the economics of the internet.
PLEASE don't let this put you off trying though, it's people trying that drive forward development!